Polyethylene terephthalate (PET) is a well-known plastic used in great quantities around the world to make products such as bottles, fibers, and packaging. PET is a polyester of terephthalic acid (TPA) and ethylene glycol. Often PET is modified with a carboxylic acid other than TPA and/or a diol other than ethylene glycol to enhance particular physical properties. For example, isophthalic acid is added as a partial replacement to terephthalic acid in commercial polyester production processes to change the morphology of the resultant polyester product. Typical modification levels are 0-5 mole % replacement of primary terephthalic acid, resulting in more desirable properties of the polyester produced. These improved properties include, for example, reduced crystallization rate and improved barrier properties for packaging resins.
As the business of manufacturing polyesters becomes more competitive, alternative manufacturing processes have become highly desirable. Manufacturing costs can be removed from the production of the polyester itself or the production of the raw materials used to make the polyester.
Commercial isophthalic acid (IPA) typically is produced using the same basic process steps as terephthalic acid, but starting with a meta-xylene feed. Commercial isophthalic acid contains low levels of impurities. Like commercial terephthalic acid, this is important for most applications where isophthalic acid is used in significant quantities. It can be appreciated that the importance of absolute purity of the isophthalic acid diminishes as the amount of isophthalic acid used is a lower overall fraction of the monomer mix for making the polyester.
The isophthalic acid production process can be divided into the oxidation zone and the hydrogenation zone. Crude isophthalic acid is produced in the oxidation zone. The crude (or non-hydrogenated) IPA is purified (or hydrogenated) in the hydrogenation zone. The hydrogenation zone typically includes the following steps: (1) replacing the mother liquor of the crude IPA-containing slurry with water, (2) heating the crude IPA/water slurry to dissolve the crude IPA in water, (3) catalytically hydrogenating the crude IPA/water solution to convert impurities to more desirable and/or easily-separable compounds, (4) precipitating the resulting purified IPA from the hydrogenated solution via multiple crystallization steps, and (5) separating the crystallized purified IPA from the remaining liquids. Although effective, this type of conventional purification process can be very expensive. Individual factors contributing to the high cost of conventional crude IPA purification methods include, for example, the heat energy required to promote dissolution of the crude IPA in water, the catalyst required for hydrogenation, the hydrogen stream required for hydrogenation, the yield loss caused by hydrogenation of some isophthalic acid, and the multiple vessels required for multi-step crystallization.
There is a need in the polyester business to decrease the costs of manufacturing PET. This invention relates to production of isophthalic acid of suitable quality without hydrogenation (i.e., non-hydrogenated IPA) for commercial use in polyester processes. Eliminating the hydrogenation zone of the IPA process significantly reduces the capital and operating costs of this PET modifier.